It confuses me that you do not think that this due diligence fee might be a peccadillo of your market and not exist in some form everywhere, although it shouldn't by this point. 

Also, the existing leases would have to be re-signed with the Mgt LLC. 

You can house hack your existing primary residence with airbnb or rent a room to someone..and then use that money to make additional payment to principal on your existing mortgage. 

Sellers can require no inspection periods right now as inventory is non existent and there’s buyers everywhere, especially on investment properties.

Obviously, each client has different risk thresholds but a new phase 1 would certainly make sense to determine to see if conditions flagged 25 years ago still exist -especially if the client otherwise really liked the deal.   

The 1% rule is almost non-existent in this market. 

My question is that: 1: I’m buying this rental property, the seller is asking to assume the existing lease but this lease doesn’t have deposit what should I do for now during this buying process and in the future? 

You can look at their true costs, the condition of the park for Capex needs, the reservations that happen, leases that may exist, get inspections - I'm sure you know all this.  

They could be fairly expensive unless they've done this many times before and are willing to simply mold an existing contract to your situation.

Phoenix is pretty saturated and very competitive.